A surveillance camera may operate for 24 hours a day, therefore, extremely bright images and extremely dark images are constantly being recorded.
The general automatic gain control of the camera often cannot keep up with an extremely large change. Also, when extremely bright and dark parts are mixed within the same picture, such as an indoor window, this cannot be dealt with by a conventional automatic gain control operation.
A method that is regulated so that brightness level I of the individual position of the image generates adjusted brightness level I′ is described in patent document 1 as an image processing method comprising the steps to process an input signal to generate an adjusted output signal.
Computational complexity is reduced by reusing the result of a part repeating before and after movement when domain operation is performed with the local domain moves and repetition.
In this way the speeding up of domain operation after the movement is disclosed in patent document 2 and patent document 3.
A line buffer is adopted for the local histogram acquisition, and a technique to store all of the local histograms before one line to this line buffer is proposed in non-patent document 1 as a local histogram average technique used for HDR (High Dynamic Range) compression.
It is disclosed in non-patent document 2 that brightness conversion can be performed by the histogram equalization method from brightness distribution information provided for each and every pixel while moving one pixel of local domain to a unit.
In the above described patent documents 1-3 and non-patent documents 1 and 2, overall throughput cannot be largely reduced. Thus this inventor suggested patent document 4. In this patent document 4, it is suggested that calculation of the brightness of the pixel of the particular position is based on an algorithm to assume the loop number of times of a pixel level.